Liquid bleaching compositions are well known in the art. Amongst the different bleaching compositions available, those relying on bleaching by hypochlorite are often preferred.
However, the drawbacks associated with the use of chlorine-based compositions are that said compositions may both damage and yellow fabrics.
We have now found that the whiteness performance and/or the fabric safety performance of a liquid aqueous hypochlorite-containing composition is mainly dependent on two factors: the pH of the bleaching environment and the presence of heavy metal ions in said bleaching environment.
It is believed that higher pH of the bleaching environment is beneficial for the whiteness and fabric safety performance because it shifts the chemical equilibrium between hypochlorite and hypochlorous acid to reduce the level of hypochlorous acid, the species which we have found to be the most responsible for the yellowing and the fabric damage.
The presence of heavy metal ions such as Ni, Co, Cu, Mn, Cr, and Fe adversely affects the hypochlorite bleaching performance. It is believed that in the washing environment said heavy metal ions catalyze the attack of hypochlorous acid on fabrics with the generation of yellow oxidized species. Said heavy metal ions also lead to reduced tensile strength of the fabrics, thereby reducing fabric resistance. It is further believed that said heavy metal ions are adsorbed per se on oxidized fabrics as colored species and catalyze the degradation of the brighteners adsorbed on fabrics generating thereby highly colored species on fabrics. Also, said heavy metal ions stabilize colored pigments of enzymatic stains such as blood or grass. Furthermore, it is believed that said heavy metal ions are detrimental for the performance of a hypochlorite composition as they tend to react with the hypochlorite present in said composition and thereby catalyze the decomposition of said hypochlorite.
In the current industrial manufacturing processes, liquid aqueous compositions comprising hypochlorite and a source of alkalinity, such as sodium hydroxide, are prepared by using the cheapest and most commonly available raw materials. Such commercially available raw materials are usually contaminated by impurities such as heavy metal ions. For example, the commercially available hypochlorite commonly used may contain proportions of other materials such as sodium carbonate and/or caustic soda and/or heavy metal ions. Also, the current caustic soda is commonly contaminated by heavy metal ions such as iron.
More particularly, in the current industrial manufacturing processes, such liquid aqueous compositions comprising hypochlorite are prepared by diluting a concentrated hypochlorite solution with tap water, i.e. non-demineralized water, in presence of said source of alkalinity, allowing precipitation of insoluble salts, such as metal hydroxides and metal carbonates, and finally removing said insoluble salts from said compositions. However, the compositions obtained are not satisfactory as regard their purity, because they still contain a certain level of heavy metal ions which have not been retained/eliminated by a conventional separation means like by filtering said compositions with a decantation filter. The amount of heavy metal ions in the finished composition is even higher when manufacturing a liquid aqueous composition comprising hypochlorite at high pH values such as 13 and above. Indeed, the solubility of most of the heavy metal ions that should be eliminated in order to ensure good whiteness and/or fabric safety performance, increases with the pH of the composition. Also the separation step, e.g. the filtration step, is quite a long operation, i.e. it is the most time consuming step in the overall manufacturing process of hypochlorite-containing compositions.
We have now found that it is essential that the levels of heavy metal ions are controlled in the liquid aqueous hypochlorite-containing compositions per se, thereby providing improved whiteness performance and/or fabric safety performance when treating fabrics with such compositions.
It is therefore an object of the present invention to provide a process for the manufacture of liquid aqueous compositions comprising hypochlorite and a strong source of alkalinity, said compositions having a particularly low content of heavy metal ions, therefore exhibiting improved whiteness and/or fabric safety performance.
It is a further object to provide such a process for the manufacture of liquid aqueous hypochlorite-containing compositions at overall reasonable costs, e.g. to provide compositions exhibiting improved whiteness and/or fabric safety performance by a process wherein the raw materials used are the cheapest and most commonly available commercially.
It has now been found that a particularly efficient control of the heavy metal ions can be achieved by using a manufacturing process where a precipitating agent is added to hypochlorite and a strong source of alkalinity in presence of tap water, so as to form precipitates which are then separated/removed from said composition. Indeed, said precipitating agent allows to precipitate the heavy metal ions present in the compositions before the separation step. The precipitating agent binds to heavy metal ions, thereby forming non-soluble species, i.e. precipitates, which are separated from the composition by any usual conventional means such as a filter during said separation step. More particularly, it has been found that the addition of said precipitating agent before the separation step to a hypochlorite-containing composition allows to remove heavy metal ions from said composition in a more efficient way as compared to the same process without the addition of said precipitating agent before the separation step. In other words, the addition of said precipitating agent before the separation step improves the efficiency of said step, even at higher pH values.
The compositions obtainable by the process of the present invention have improved physical and chemical stability. A further advantage of the compositions obtainable by the process of the present invention is that in addition to their outstanding whitening action said compositions allow also good stain removal.